JPS62194212A - Objective for optical information reader - Google Patents

Abstract

PURPOSE:To compensate aberrations excellently even when divergent light is made incident by providing an objective with two refracting surfaces expressed by a general aspherical surface expansion expression containing a term which is proportional to the value obtained by raising the height from the optical axis of the incident light beam to the 10th power, and satisfying specific conditions. CONSTITUTION:The objective 10 has two refracting surfaces having a common optical axis and those two refracting surfaces are so formed as to satisfy an inequality II where A<(1)>4 and A<(1)>6 are aspherical surface coefficients A4 and A6 of the 1st refracting surface S1 while the distance X from a tangential plane at the peak point of a refracting surface at a point with height Y from the optical axis on the refracting surface is expressed by an aspherical surface expansion equation I where A4-A10 are aspherical surface coefficients. Here, C is the curvature of the refracting surface at the peak point, K, A4, A6, A8, and A10 constants, (n) a refractive index, and (f) the focal length. Consequently, when this lens is used for a finite conjugate optical pickup, the objective for an optical information reader which compensate aberrations excellently is obtained.

Description

[Detailed description of the invention] Flame Five Dukes 1 The present invention relates to an objective lens for an optical information reading device.

A large-diameter aspherical lens as shown in FIG. 7, which is suitable for use as an objective lens in an optical information reading device such as the Bidan Optical Video Disc Brake A7, is described in Japanese Patent Application Laid-open No. 76512/1983. . In FIG. 7, a laser beam emitted from a light source such as a laser tube (not shown) is refracted by an objective lens 1, passes through a substrate 2a of the recording disk 2, and then is deposited on a recording surface 2b for information detection. It becomes a spot light. Then, the recorded information is read by detecting a change in the amount of transmitted light or reflected light from the recording surface 2b. In the objective lens 1, the first refractive surface S1 onto which the laser beam is incident is an aspherical surface expressed by an aspherical expansion formula as shown in the following equation.

C+ Y2 +Aa Y' +As Y →-AaY...(
1) Here, X is the distance from the tangential plane at the apex of the first refractive surface S1 of the point whose height from the optical axis on the first refractive surface S1 is Y, and C1 is the curvature at the apex of the first refractive surface S+. , Y is the height from the optical axis, and K+, A, +, A6°A8 are the aspheric coefficients.

Further, the second refractive surface 82 of the objective lens 1 facing the recording disk 2 is a hyperbolic surface expressed by an aspherical expansion formula as shown in the following equation.

C2Y'...(2) Here, C2 is the curvature at the apex of the second refractive surface S2, and K
2 is the aspheric coefficient of the second refractive surface.

Now, the aspherical coefficients are +, AJ, As, As, and 2.
are, for example, values shown in the following equations.

K+ =-2,41688・・・・・・(3)A
4 =0.62875X10-2・・・・・・・・・(
4) A6=0.21838X10-3・・・・・・・・・
・(5) As −0,67164X 10 − (6)
K2 = 149.999 ・・・・・・・・・・・・
......(7) Also, the radius of curvature r+ (=1/C+
) and C2 (=1/Cz), the distance between each vertex of the first and second refractive surfaces S1 and S2, that is, the thickness d of the objective lens,
The distance between the vertex of the second refractive surface S2 and the recording disk 2, that is, the working distance WD, and the refractive index n1 of the objective lens 1 have, for example, values shown in the following equations.

r+=3.3710(m) −・=(8)j'2=
14.768 (number) ・・・・・・(9) d=4.
0 (s) ... (10) WD=1.
400 (rrvR)...111)n+=
1.70214 +++++++ (12) In addition, 10
Since a resolving power of approximately 0.00 lines/M is required, the numerical apertures N and A are approximately 0.45 to 0.5, and aberration correction is performed so that residual aberrations are kept within the diffraction limit.

The conventional objective lens as described above can be used in an optical pickup having a complicated structure including a collimator lens that converts laser light into parallel light, as shown in FIG. That is,
A laser beam emitted from a light source 3 is collimated by a collimator lens 4 and then focused onto the recording surface of a recording disk 2 via a beam splitter 5 and an objective lens 1.
The reflected light from the recording surface is focused by the objective lens 1, reflected by the beam splitter 5, and incident on the detector 8 via the plano-convex lens 6 and the cylindrical lens 7. By doing this, the objective lens is formed of a single lens, which makes it small and lightweight, so that the objective lens can be well controlled by a focus servo device or a tracking servo device configured to drive the objective lens according to an error signal. That will happen. However, conventional objective lenses are designed to perform good aberration correction only when parallel rays are incident, so the laser beam from the light source 3 is reflected by a half mirror 9 and diverged as shown in FIG. The light enters the objective lens 1 as it is and focuses it on the recording surface of the recording disk 2, and the reflected light from this recording surface is passed through the objective lens 1.
It could not be used in a finite conjugate optical pickup in which the light is focused by the finite mirror 9 and then incident on the detector 8 via the half mirror 9. Therefore, the conventional optical information reading device using an objective lens has a complicated structure.

An object of the present invention is to provide an objective lens for an optical information reading device that can perform good aberration correction when used in a finite conjugate optical pickup.

The objective lens of the optical information reading device according to the present invention has two refractive surfaces having a common optical axis, and these two refractive surfaces are
When A4~AID is the aspheric coefficient, Y2 X=□→ As Y' 1+ 1-KC" +As Y' +As Y +A+oY...
(13) When the aspheric coefficient A4 of the refractive surface S1 is As, -K + → K2
drl
(n-1)f... (14) It is formed so that it may satisfy.

X-product-1 Examples of the present invention will be described below with reference to FIGS. 1 to 6.

In FIG. 6, an objective lens according to the invention and a recording disk 2 are arranged in the same manner as the objective lens 1 and recording disk 2 in FIG. The first refractive surface S+ on which the laser beam of the objective lens 3 is incident and the recording disk 2
The 21i1i'1 folding plane S2 facing is both (13
) is an aspheric surface expressed by an aspheric expansion formula such as the following. Furthermore, the condition shown in equation (14) is satisfied.

Equation (14) shows the conditions for suppressing coma aberration occurring off-axis to a low level and also suppressing eccentric coma caused by axis deviation of the spherical surface. By satisfying this condition, it is possible to obtain a lens that has good off-axis performance and is easy to manufacture, with less deterioration in performance due to axis deviation of the spherical surface that occurs during manufacturing.

Next, assuming that it is used in a finite conjugate optical pickup, the distance between the objective lens and the light source is set to 20 m, and diverging light is incident on the objective lens, and 1000 beams/#l111
Since a certain level of resolution is required, the NA of the recording disk should be set to 0.
45, thickness t=1.20 and refractive index n'=1
．． The table below shows a specific example of numerical values when axial aberrations are corrected including the aberrations caused by the recording disk 2 of No. 55.

Aspheric coefficient A4 of first refractive surface S1. A6. A8゜A+o
The value of A4. As, Aa, A+o are the aspherical coefficients A4 and 6 of the second refractive surface S2. 8a, the value of Ato.

In numerical example 1, the spherical aberration and sine conditions are shown in Figure 3 (A).
The coma aberration becomes as shown in FIG. 2B. Similarly, in the numerical example (3), the spherical aberration and sine conditions are as shown in FIG. 4(A), and the comatic aberration is as shown in FIG. 4(B). In addition, in the numerical example (■), the spherical aberration and sine condition are as shown in Figure 5<A), and the coma aberration is as shown in Figure 5 (<A).
As shown in B).

From Figures 3 to 5, it can be seen that in all of the numerical examples (■ to ■), sufficient aberrations are corrected including the substrate of the recording disk, and off-axis aberrations are also corrected to the extent necessary for practical use. Ru.

In addition, the degree of performance deterioration with respect to the center thickness error of the lens in numerical examples 1 to ■ is shown in Fig. 5 by solid lines ■, ■, and ■, respectively.
Displacement of the center axis of the artistic sphere, or deCenterin
Figure 6 shows the degree of performance deterioration for () with solid lines ■, ■, ■
Indicated by Since the wavefront aberration is allowed to deteriorate up to about 0.07λ, it can be seen from FIGS. 5 and 6 that the tolerance for center thickness error and center axis deviation is sufficiently large, and productivity is good.

As detailed above, the objective lens of the optical information reading device according to the present invention is expressed by a general aspheric expansion formula including a term proportional to the tenth power of the height of the incident light ray from the optical axis. has two refractive surfaces, and (-)/r+10OA+1000A
Since it satisfies the condition <(d/(n-1)f), aberrations can be corrected well even when diverging light is incident, and it can be used as an objective lens for a finite conjugate optical pickup. I can do it. Therefore, according to the present invention, it is possible to simplify the configuration of the optical information reading device and at the same time, perform good tracking control and focus control.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing one embodiment of the present invention, FIGS. 2 to 4 are graphs showing the characteristics of numerical examples 1 to 2, and FIG. Figure 6 is a graph showing the change in wavefront aberration with respect to center thickness error in ~■, Numerical Example 1
A graph showing the change in wavefront aberration with respect to the aspheric center axis deviation in ~■, Fig. 7 is a cross-sectional view showing a conventional objective lens, Fig. 8 is a diagram showing an example of an optical pickup, Fig. 9 is FIG. 7 is a diagram showing another example of an optical pickup.

Claims (1)

[Claims] An objective lens for an optical information reading device having two refractive surfaces having a common optical axis, wherein the two refractive surfaces are: X: the height of the refractive surface from the optical axis; Distance of the point Y from the tangent plane at the apex of the refracting surface Y: Height from the optical axis C: Curvature K at the apex of the refractive surface, A_4, A_6, A_8, A_1_0: When set as a constant, X= CY^2/(1+√[1-KC^2Y^2])+A
_4Y^4+A_6Y^6+A_8Y^8+A_1_0
It is expressed by the aspherical expansion formula Y^1^0, where n: refractive index f: focal length r_1: curvature at one vertex of the two refractive surfaces K_1: value of the one constant K K_2: the above Value of the constant K of the other of the two refractive surfaces A_4: Value of the coefficient A_4 of the one A_6: Value of the coefficient A_6 of the one of the two refractive surfaces ▲There are mathematical formulas, chemical formulas, tables, etc.▼ It is characterized by satisfying the following. An objective lens for an optical information reader.